Precursor and method for preparing Ni based cathode material for rechargeable lithium ion batteries

ABSTRACT

A crystalline precursor compound for manufacturing a lithium transition metal based oxide powder usable as an active positive electrode material in lithium-ion batteries, the precursor having a general formula Li 1−a ((Ni z (Ni 1/2  Mn 1/2 ) y Co x ) 1−k  A k ) 1+a O 2 , wherein x+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and 0.03≤a≤0.35, wherein the precursor has a crystalline size L expressed in nm, with 15≤L≤36. Also a method is described for manufacturing a positive electrode material having a general formula Li 1+a ′M′ 1−a −O 2 , with M′=(Ni z (Ni 1/2  Mn 1/2 ) y CO x ) 1−k  A k , wherein x+y+z=1.0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and 0.01≤a′≤0.10, by sintering the lithium deficient precursor powder mixed with either one of LiOH, LiOH.H 2 O, in an oxidizing atmosphere at a temperature between 800 and 1000° C., for a time between 6 and 36 hrs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of InternationalPatent Application No. PCT/EP2018/053638, filed on Feb. 14, 2018, whichclaims the benefit of European Patent Application No. 17159083.9, filedon Mar. 3, 2017, the entire contents of which are all herebyincorporated herein by reference.

TECHNICAL FIELD AND BACKGROUND

This invention relates to a precursor of and a method to prepareNi-excess “NMC” cathode powdery material on a large-scale and at lowcost. By “NMC” we refer to lithium-nickel-manganese-cobalt-oxide. TheNi-excess NMC powder can be used as a cathode active material in Li-ionrechargeable batteries. Batteries containing the cathode materials ofthe invention enhance their performances, such as providing a highercycle stability and a low content of soluble base.

The global market of lithium-ion batteries (LIB) has been concentratingon large batteries. The term “large batteries” refers to applications inelectric vehicles (EV), as well as in stationary power stations. TheseEV or large stationaries require much larger power sources than thepreviously dominating batteries for portable devices such as laptops,smartphones, tablets, etc. Therefore, there are fundamentally differentrequirements for the “large battery” cathode materials, not onlyperformance-wise, but also from the point of view of resource scarcity.Previously, LiCoO₂ (LCO) was used as the cathode material for mostrechargeable lithium batteries. However, LCO is not sustainable for thelarge batteries due to the limited cobalt resources—as approximately 30%of the cobalt production worldwide is currently already used forbatteries, according to the Cobalt Development Institute. Therefore,Lithium Nickel-Cobalt-Manganese-based Oxide, having roughly thestoichiometry LiM′O₂, where M′=Ni_(x′)Mn_(y′)Co_(z′) (when not doped)has become a promising alternative cathode material due its lesscritical resources situation. This material has excellent cyclingproperties, long-life stability, high energy density, good structuralstability, and low cost. Various compositions of NMC have been developedto improve the energy density of NMC by relatively increasing the amountof Ni, without losing its advantages mentioned before. Typical NMC basedmaterials are “111”, “442”, “532”, and “622”: “111” withM′=Ni_(1/3)Mn_(1/3)Co_(1/3), “442” with M′=Ni_(0.4)Mn_(0.4)Co_(0.2),“532” with M′=Ni_(0.5)Mn_(0.3)Co_(0.2), “622” withM′=Ni_(0.6)Mn_(0.2)Co_(0.2). The NMC cathode materials contain lesscobalt because it is replaced by nickel and manganese. Since nickel andmanganese are cheaper than cobalt and relatively more abundant, NMCpotentially replaces LiCoO₂ in the large batteries.

NMC cathode materials can roughly be understood as a solid statesolution of LiCoO₂, LiNi_(0.5)Mn_(0.5)O₂ and LiNiO₂, corresponding tothe general formula Li_(1+a)[Ni_(z)(Ni_(1/2) Mn_(1/2))_(y)Co_(x)]_(1−a)O₂, where “z” stands for the Ni(3+) excess, as in LiNi_(0.5)Mn_(0.5)O₂Ni is divalent and in LiNiO₂ Ni is trivalent. At 4.3 V the nominalcapacity of LiCoO₂ and LiNi_(0.5)Mn_(0.5)O₂ is about 160 mAh/g, against220 mAh/g for LiNiO₂. The reversible capacity of any NMC compound can beroughly estimated from these capacities. For example NMC 622 can beunderstood as 0.2 LiCoO₂+0.4 LiNi_(0.5)Mn_(0.5)O₂+0.4 LiNiO₂. Thus theexpected capacity equals 0.2×160+0.4×160+0.4×220=184 mAh/g. The capacityincreases with “Ni excess”, where “Ni excess” is the fraction of3-valent Ni; for example in NMC 622 the Ni excess is 0.4 (if we assumelithium stoichiometry with Li:(Ni+Mn+Co)=1.0). Obviously the capacityincreases with Ni excess, so that at the same voltage, Ni-excess NMCpossesses a higher energy density than LCO, which means less weight orvolume of cathode material is required for a certain energy demand whenusing Ni-excess NMC instead of LCO. Additionally due to the lower priceof nickel and manganese—compared to cobalt—the cost of cathode per unitof delivered energy is much reduced. Thus, the higher energy density andlower cost of Ni—excess NMC—by contrast to LCO—is more preferred in the“large battery” market.

There are two major trends to achieve a high energy density. One trendis to increase the Ni excess up-to very high values. InNCA—LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, for example, the Ni excess is veryhigh; it is 0.8 as all Ni is 3 valent. In NC91—LiNi_(0.9)Co_(0.1)O₂ theNi excess is even 0.9. These cathodes have very high capacities even atrelatively low charge voltage. As an example—NC91 has capacities as highas 220 mAh/g at 4.3V. These cathodes have a major disadvantage: if thebattery is fully charged and the cathodes are in the delithiated state,the values of “x” in the resulting Li_(1−x)MO₂ are high. These highlydelithiated cathodes are very unsafe when in contact with electrolyte.Once a certain temperature in the battery has been reached the cathodesdecompose and deliver oxygen which combusts the electrolyte. Basicallythe electrolytes reduce the cathode. After the reaction—as there islarge Ni-excess—most of the transition metal is 2 valent.Schematically—each mol of cathode can deliver one mol oxygen to combustthe electrolyte: NiO₂+electrolyte→NiO+{H₂O, CO₂}. The safety issue ofbatteries is mostly caused by the electrolyte combustion heat.

The other trend to achieve a high energy density is to increase the Niexcess towards intermediate values. Typical values for the Ni excessrange from about 0.25 to about 0.6. This region we will refer as “highNi-excess”. The current invention refers to a process to prepare NMCwith high Ni-excess. The capacity at 4.2 or 4.3V of high Ni-excess NMCis less than that of “very high” Ni-excess compound (with Niexcess >0.6). However, the capacity can also be increased by increasingthe charge voltage. The resulting delithiated cathodes are safer thanthe delithiated very high Ni-excess cathodes mentioned above. Whereas Nitends to form NiO, Ni—M′ tends to form stable M′₃O₄ compounds. Thesecompounds have a higher final oxygen stoichiometry thus less oxygen isavailable to combust the electrolyte. As a result, the safety of highNi-excess cathodes is improved even if a higher charge voltage isapplied.

The prior art teaches that the cycle stability of NMC at high voltagemay be insufficient, however, it can be improved by applying a surfacecoating, as disclosed e.g. in WO2016-116862. The surface coatingbasically stabilizes the surface against unwanted side reactions betweenelectrolyte and cathode during cycling.

As the capacity of NMC material increases with Ni-excess, “Ni-excess”NMC cathode materials, like NMC 532 and NMC 622, possess a highercapacity in batteries than with less Ni, as for example NMC 111 (havinga Ni excess=0). However, the production becomes more and more difficultwith increasing Ni content. As an example—very high Ni-excess cathodematerials like NCA cannot be prepared in air or using Li₂CO₃ as alithium source.

Because of the low thermodynamic stability of Li in Ni excess material,“soluble bases” occur easily on the surface of the final product, theconcept of “soluble base” being explicitly discussed in e.g.WO2012-107313: the soluble base refers to surface impurities likelithium carbonate (Li₂CO₃) and lithium hydroxide (LiOH). These solublebases are a concern since especially residual Li₂CO₃ causes poor cyclingstability in the lithium ion battery. Therefore, the preparation of veryhigh Ni-excess cathode materials is performed in CO₂ free oxidizing gas(typically oxygen) to reduce the soluble base content at increasingtemperature, and LiOH is used as a lithium source instead of Li₂CO₃.Contrary to this, the low Ni NMC 111 can easily be prepared in normalair and using a low-cost Li₂CO₃ precursor.

The preparation of NMC 532 (having a Ni excess=0.2) is more difficultthan NMC 111, but NMC 532 can still be processed at large-scale througha low-cost and simple solid state reaction under air. Thisprocess—referred to as “direct sintering”—is the firing of a blend of amixed metal precursor (for example M′(OH)₂ precursor) and a lithiumsource. The lithium source is preferably Li₂CO₃, as in the production ofNMC 111, due to its low price.

Another promising Ni-excess compound is NMC 622, whose Ni excess is 0.4and its capacity is higher than that of NMC 532. However, compared toNMC 532 and NMC 111, it is very difficult to prepare NMC 622 with lowsoluble base using a large-scale and low cost process such as directsintering. As discussed in U.S. Pat. No. 7,648,693, these bases may comefrom unreacted Li₂CO₃ present in the reagents of the lithium sources,usually Li₂CO₃ or LiOH.H₂O, where LiOH.H₂O normally contains 1 wt %Li₂CO₃ impurity. These bases can also originate from the mixedtransition metal hydroxides that are used as the transition metal sourcein the production. The mixed transition metal hydroxide is usuallyobtained by co-precipitation of transition metal sulfates and anindustrial grade base such as NaOH. Thus, the hydroxide can contain aCO₃ ²⁻ impurity. During sintering with a lithium source the CO₃ ²⁻residual reacts with lithium and creates Li₂CO₃. As during sinteringLiM′O₂ crystallites grow, the Li₂CO₃ base will be accumulated on thesurface of these crystallites. Thus, after sintering at high temperaturein a high Ni-excess NMC, like NMC 622, carbonate compounds remain on thesurface of the final product. This base can dissolve in water, andtherefore the soluble base content can be measured by a technique calledpH titration, as discussed in U.S. Pat. No. 7,648,693.

The presence of soluble base in the final NMC material causes a seriousgas generation in full cells, which is usually called “bulging”. Thismay result in a poor cycle life of the battery, together with safetyconcerns. Therefore, in order to use Ni-excess NMC materials for largebattery applications, an effective and low-cost processing method isnecessary that avoids the formation of such high soluble base contents.

The direct sintering method mentioned before is performed in trays in acontinuous manner. “Trays” are ceramic vessels which contain the blendor product during sintering, they are sometimes also referred to as“saggers”. The trays are continuously fed to a furnace, and during themovement through the conveyor furnace the reaction towards the finalsintered LiM′O₂ proceeds. The sintering cost depends strongly on thethroughput of the sintering process. The faster the trays move acrossthe furnace (referred to as the “sintering time”) and the more blend thetrays carry (referred to as the “tray load”), the higher the throughputof the furnace is. Moreover, the furnace has a high investment cost.Therefore, if the throughput is small, the furnace depreciation andoperating cost significantly contributes to the total process cost. Inorder to reduce manufacturing cost, a high throughput is thus desired.

Many large-scale direct sintering production methods for Ni-excess NMChave been tried. As the Ni-excess increases, the direct sinteringbecomes more difficult. It is observed that high Ni-excess NMC requireslong sintering times and a low tray load to be successful. Since highNi-excess NMC has a too low “tray throughput”, the direct sinteringproduction is not available to produce a high quality material at anacceptable low cost. For example, when using a Li₂CO₃ precursor, thethroughput limitation can be traced back to the relatively highthermodynamic stability of Li₂CO₃ causing slower reaction kinetics whenthe reaction proceeds. The mechanism that slows down the reaction speedis a gas phase limitation, since due to a low CO₂ equilibrium partialpressure, the removal of CO₂ hinders the reaction. Therefore theapplication of other lithium sources having a lower thermodynamicstability could solve this issue. LiOH.H₂O is such a precursor and thecorresponding H₂O equilibrium partial pressures are higher than those ofCO₂. Thus LiOH.H₂O is widely applied as a precursor for direct sinteringhigher Ni containing cathode materials. This typical process to preparehigh Ni-excess NMC is for example applied in US 2015/0010824. LiOH.H₂Owith a low Li₂CO₃ impurity as a lithium source is blended with the mixedtransition metal hydroxide at the target composition, and sintered athigh temperature under an air atmosphere. In this process, the basecontent of such high Ni-excess NMC final product (like NMC 622) is muchreduced.

However, LiOH.H₂O makes excess vapor during heating and sintering steps,resulting in various problems. For example, LiOH.H₂O has a low meltingpoint of about 400° C. At that temperature the reactivity of a metalprecursor (like M′OOH) with LiOH.H₂O is not high. As a result moltenLiOH.H₂O is present at the same time as large amounts of H₂O vapor arecreated. These vapor streams physically cause a de-mixing of the blend,resulting in a final product having an inhomogeneous chemicalcomposition, where especially the Li:M stoichiometric ratio will varywithin a tray. The larger the tray load is, the more severe this issuebecomes. Additionally, there is also a heat limitation issue. If thetray load is high then the blend in the center of the trays will be lesssintered. Thus—at high tray throughput—inhomogeneously sintered productwill be achieved. The larger the tray load is, the more severe theseissues become.

For a high quality final cathode with a homogeneous composition, thevariation of Li:M composition and sintering degree of particles withinthe powder needs to be limited. Therefore, in order to achieve a highquality product a low tray load is required. If we compare the directfiring using Li₂CO₃ precursor and LiOH.H₂O precursor then the LiOH.H₂Oallows a higher tray throughput, but in an economical mass productionprocess the tray throughput must still be higher. The tray basedconveyor furnace consists of a continuous firing kiln with a motordriven roller way, which is good for consistent high-volume productionof NMC. However, generally there is a heat transfer limitation of theblend in the trays because both tray and product are good heatinsulators, resulting in an inhomogeneous state of sintering in thefinal product. Furthermore, this heat transfer issue will be more severeif the firing time is reduced in order to increase throughput.Therefore, improved sintering methods to enhance the transport of heatwithin the blend are necessary for a large-scale preparation of highquality NMC.

Rotary furnace technology provides a faster transport of heat within theblend. It also prevents de-mixing of the blend. A typically usedindirect-fired sintering rotary furnace is basically a metallic rotatingtube which is heated from the outside, such as disclosed in U.S. Pat.No. 7,939,202. Cold blend or product is transported towards the hot zoneof the tube, and within the tube the blend or product continuouslymoves, and is continuously heated, thereby preventing a de-mixing whichprevents an inhomogeneous Li:M stoichiometric ratio. Thus a rotaryfurnace has much less heat transfer limitations, and provides a muchhigher throughput and has a low operating cost per production capacityfor an intrinsically lower investment cost. Rotary furnaces are alsovery compact and allow to increase production capacity without need touse more land. However, as said before, direct sintering requiresrelatively long sintering times and a relatively high temperature, (forexample exceeding 800° C. for NMC 622) to obtain a high quality product.It is difficult to obtain a long sintering time in a rotary furnace.Also, at the high sintering temperature the lithium in a lithiatedtransition metal oxide will react with the tube material and cause tubecorrosion. Therefore, indirect-fired sintering rotary furnaces are notsuitable for direct sintering.

Besides direct sintering also split firing has been proposed. U.S. Pat.No. 7,648,693 proposes a split method, where the firing is conducted intwo steps: a first lithiation at relatively low temperature, like 700°C., and a second step of sintering at a higher temperature. In thispatent, a large-scale preparation of LiMO₂ withM=Ni_(4/15)(Mn_(1/2)Ni_(1/2))_(8/15)Co_(0.2) is achieved with a finalproduct that is almost free of soluble base, resulting in improvedcycling stability. The split method could thus be a potential way toprepare e.g. NMC 622 free of soluble base and at a low cost. In thesplit method all lithium is added to the blend before the 1^(st)sintering. Under such conditions it is practically impossible to fullyreact the metal precursor with the Li₂CO₃ at reasonable high throughput.Therefore, the split method is not usable for the large-scale productionof NMC 622 with Li₂CO₃ as a lithium source because excessive amounts ofpreheated air have to be pumped through the reactor. Practically thesplit method is limited to lower Ni-excess NMC, such as NMC 532.

A further variation of the split method is suggested in U.S. Pat. No.9,327,996 B2. The method for producing NMC—as is disclosed in U.S. Pat.No. 9,327,996 B2—provides a step of firing a lithium-containingcarbonate blend in a rotary furnace to produce a lithiated intermediateproduct. Rotary firing gives a large benefit for the 1^(st) sintering.It allows for lower cost and excellent production efficiency. However,in case of high Ni-excess NMC, the preparation of a fully lithiatedintermediate product is not possible because it is impossible to finishthe lithiation reaction when using Li₂CO₃ as a lithium source. Thus theresidual Li₂CO₃ content after the 1^(st) sintering is too high. During a2^(nd) sintering at high throughput it will be practically impossible toremove sufficient Li₂CO₃ thus the soluble base content of the finalproduct will be too high. The final product will have a poor performancedue to high bulging and poor cycling stability.

Therefore, the object of the present invention is to provide a low-costand efficient manufacturing process making use of an inventiveintermediate product, to supply lithium transition metal oxide cathodematerials having a Ni excess, and especially suitable for higher voltagebattery applications where the charge voltage is at least 4.3V.

SUMMARY

Viewed from a first aspect, the invention can provide a crystallineprecursor compound or intermediate product for manufacturing a lithiumtransition metal based oxide powder usable as an active positiveelectrode material in lithium-ion batteries, the precursor having ageneral formula Li_(1−a)((Ni_(z)(Ni_(1/2) Mn_(1/2))_(y)Co_(x))_(1−k)A_(k))_(1+a) O₂, wherein x+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant,0≤k≤0.1, and 0.03≤a≤0.35, wherein the precursor has a crystalline size Lexpressed in nm, with 15≤L≤36. The crystalline precursor may have aLi₂CO₃ content <0.4 wt %. In an embodiment, 0.35≤z≤0.50 and 0.05≤a≤0.30.In another embodiment, 0.15≤x≤0.25. The crystalline precursor compoundof the previous embodiments may have an integrated intensity ratioI003/I104<1, wherein I003 and I104 are the peak intensities of the Braggpeaks (003) and (104) of the XRD pattern of the crystalline precursorcompound. Also, the precursor compound may have an integrated intensityratio I003/I104<0.9. In another embodiment, the precursor compound mayhave a ratio R of the intensities of the combined Bragg peak (006, 102)and the Bragg peak (101) with R=((I006+I102)/I101) and 0.5<R<1.16. Inall of these embodiments, the precursor may also have a crystalline sizeL expressed in nm, with 25≤L≤36. In different embodiments of theinvention, A is either one or more of the elements of the groupconsisting of Al, Ti, Mg, W, Zr, Cr and V. A dopant, also called adoping agent, is a trace impurity element that is inserted into asubstance (in very low concentrations) in order to alter the propertiesof the substance. The advantage of having dopants can be eitherimprovement of structural and thermal stability or enhancement of thelithium ionic conductivity. It might also be that oxygen in the generalformula is partly replaced by S, F or N.

Viewed from a second aspect, the invention can provide a method forpreparing a positive electrode material having the general formulaLi_(1+a′)M′_(1−a′) O₂, with M′=(Ni_(z)(Ni^(1/2)Mn_(1/2))_(y)Co_(x))_(1−k) A_(k), wherein x+y+z=1, 0.1≤x≤0.4,0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and 0.01≤a′≤0.10, the methodcomprising the steps of:

-   -   providing a M′-based precursor prepared from the        co-precipitation of metal salts with a base;    -   mixing the M′-based precursor with either one of LiOH, Li₂O and        LiOH.H₂O, thereby obtaining a first mixture, whereby the Li to        transition metal ratio in the first mixture is between 0.65 and        0.97,    -   sintering the first mixture in an oxidizing atmosphere in a        rotary kiln at a temperature between 650 and 850° C., for a time        between ⅓ and 3 hrs, thereby obtaining the lithium deficient        precursor powder (or intermediate product) of the first aspect        of the invention,    -   mixing the lithium deficient precursor powder with either one of        LiOH, Li₂O and LiOH.H₂O, thereby obtaining a second mixture, and    -   sintering the second mixture in an oxidizing atmosphere at a        temperature between 800 and 1000° C., for a time between 6 and        36 hrs, whereby the positive electrode material having the        general formula Li_(1+a′)M′_(1−a′) O₂ as defined above is        obtained. In a particular method embodiment, a method is        provided for preparing a positive electrode material comprising        a core material having the general formula Li_(1+a′)M′_(1−a′)        O₂, with M′=(Ni_(z)(Ni_(1/2) Mn_(1/2))_(y)Co_(x))_(1−k) A_(k),        wherein x+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1,        and 0.01≤a′≤0.10, and a coating comprising a metal M″-oxide, the        method comprising the steps of the method mentioned before for        providing the core material, and additionally the steps of        either:        A1) providing a third mixture comprising the core material being        obtained by the method mentioned before and a compound        comprising M″, and        A2) heating the third mixture to a sintering temperature between        600° C. and 800° C.; or        B1) providing a fourth mixture comprising the core material        being obtained by the method mentioned before, a        fluorine-containing polymer and a compound comprising M″, and        B2) heating the fourth mixture to a sintering temperature        between 250 and 500° C., or        C1) providing a fifth mixture comprising the core material        obtained by the method mentioned before, an inorganic oxidizing        chemical compound, and a chemical that is a Li-acceptor, and        C2) heating the fifth mixture at a temperature between 300 and        800° C. in an oxygen comprising atmosphere. In this step the        heating temperature may be limited to 350 to 450° C.

In this particular method the compound comprising M″ in either one ofsteps A1) and B1) may be either one or more of an oxide, a sulfate, ahydroxide and a carbonate, and M″ may be either one or more of theelements Al, Ca, Ti, Mg, W, Zr, B, Nb and Si. In particular, it may beAl₂(SO₄)₃. Also in this method, the compound comprising M″ in either oneof steps A1) and B1) may be a nanometric alumina powder having a D50<100nm and a BET≤50 m²/g. Also, the fluorine-containing polymer provided instep B1) may be either one of a PVDF homopolymer, a PVDF copolymer, aPVDF-HFP polymer (hexa-fluoro propylene) and a PTFE polymer, and whereinthe amount of fluorine-containing polymer in the fourth mixture isbetween 0.1 and 2 wt %. In this method also, in step C1) the inorganicoxidizing chemical compound may be NaHSO₅, or either one of a chloride,a chlorate, a perchlorate and a hypochloride of either one of potassium,sodium, lithium, magnesium and calcium, and the Li-acceptor chemical maybe either one of AlPO₄, Li₃AlF₆ and AlF₃. More preferably, both theinorganic oxidizing chemical compound and the Li-acceptor chemical maybe the same compound, being either one of Li₂S₂O₈, H₂S₂O₈ and Na₂S₂O₈.In this method also, in step C1) a nanosized Al₂O₃ powder may beprovided as a further Li-acceptor chemical.

In an embodiment of the different methods, in the rotary kiln an airflow is applied between 0.5 and 3.5 m³/kg, and preferably between 1.0and 2.5 m³/kg. In another embodiment of the different methods, the stepof sintering the second mixture may be performed in a tray conveyorfurnace wherein each tray carries at least 5 kg of mixture. Also, it maybe preferred that between the step of providing a M′-based precursor andthe step of mixing the M′-based precursor with either one of LiOH, Li₂Oand LiOH.H₂O the M′-based precursor is subjected to a roasting step at atemperature above 200° C. in a protective atmosphere, such as under N₂.In some embodiments, after this roasting step the transition metals inthe M′-based precursor have a mean oxidation state >2.5 and a content ofH₂O<15 wt %. Also, after this roasting step the transition metals in theM′-based precursor may have a mean oxidation state >2.7 and a content ofH₂O<5 wt %.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Process flow chart of Manufacturing Example 3

FIG. 2: Lithium carbonate content of NMC samples prepared using directand double sintering

FIG. 3: XRD patterns of the pretreated transition metal samples

FIG. 4: XRD patterns of the intermediate and final product of NMCsamples prepared using double sintering

FIG. 5: XRD patterns of the intermediate and final product of NMCsamples prepared using roasted transition metal source and doublesintering

FIG. 6: XRD patterns of intermediate product with low ratio of Li:M ofNMC samples

FIG. 7: Coin cell profile of NMC samples prepared using the intermediateproduct with low ratio of Li:M

FIG. 8: Coin cell profile of Al and Al/F coated NMC samples

FIG. 9: Coin cell profile of Al and Al/F coated NMC samples preparedusing roasted transition metal source

FIG. 10: Total base content of NMC samples prepared using various airflow conditions during the 1^(st) sintering

FIG. 11: Li/metal content of NMC samples prepared using various air flowconditions during the 1^(st) sintering

DETAILED DESCRIPTION

The current patent discloses an improved split firing method where the1^(st) sintering is done using a rotary furnace resulting in a lithiumdeficient intermediate NMC which is sintered in a 2^(nd) sintering. Theuse of a lithium deficient intermediate increases the reaction ratebetween the mixed metal precursor and the lithium source. Thus thetemperature can be lowered. The lithium deficient product is lessreactive so corrosion of the metal tube is reduced. To produce NMC withgood quality and high throughput, a double sintering method isconducted. First, the mixed transition metal source is blended with asource of Li and then sintered. At this step, the Li source provides aLi-deficient stoichiometry, meaning that the ratio of Li to transitionmetal (Li:M) in LiMO₂ is less than 1. Then, in the 2^(nd) sintering thelithium deficient sintered precursor is mixed with LiOH.H₂O in order tocorrect the ratio of Li:M to the final target composition. Inconsequence, high Ni-excess NMC with a low soluble base content isobtained on a large-scale production through the double sintering methodwhich uses a lithium deficient sintered precursor.

MANUFACTURING EXAMPLE 1 Prior Art—Counterexample

The following description gives an example of the standard manufacturingprocedure of NMC powders when applying a conventional direct sinteringprocess which is a solid state reaction between a lithium source,usually Li₂CO₃ or LiOH.H₂O, and a mixed transition metal source, usuallya mixed metal hydroxide M′(OH)₂ or oxyhydroxide M′OOH (with M′=Ni, Mnand Co), but not limited to these hydroxides. In a typicalconfiguration, the direct sintering method comprises the followingsteps:

1) Blending of the mixture of precursors: the lithium source and themixed transition metal source are homogenously blended in a HenschelMixer® for 30 mins by a dry powder mixing process,

2) Sintering the blends in trays: the powder mixture is loaded in traysand sintered at 900° C. for 10 hours under dry air atmosphere in achamber furnace. The dry air is continuously pumped into the equipmentat a flow rate of 20 L/hr.

3) Post-treatment: after sintering, the sintered cake is crushed,classified and sieved so as to obtain a non-agglomerate NMC powder.

The direct sintering is generally conducted in a tray based furnace. Forreducing the heat transfer limitation which causes inhomogeneousdistribution of Li component and poor electrochemical performance a lowtray load is required. The invention observes that the direct sinteringmethod is not applicable for the large-scale production of mostNi-excess NMC material (having a Ni-excess >0.25) that does not containan excess of soluble base.

MANUFACTURING EXAMPLE 2 Counterexample

This example provides a lithium deficient sintered precursor to preparehigh Ni-excess NMC on a large-scale by double sintering. The processincludes among others two sintering steps:

1) 1^(st) blending: to obtain a lithium deficient sintered precursor,Li₂CO₃ and the mixed transition metal source are homogenously blended ina Henschel Mixer® for 30 mins.

2) 1^(st) sintering: the mixture from the 1^(st) blending step issintered at 900° C. for 10 hours under dry air atmosphere in a traybased furnace. The dry air is continuously pumped into the equipment ata flow rate of 40 L/hr. After the 1^(st) sintering, the sintered cake iscrushed, classified and sieved so as to ready it for the 2^(nd) blendingstep. The product obtained from this step is a lithium deficientsintered precursor, meaning that the Li:M′ stoichiometric ratio inLiM′O₂ is less than 1. The composition of this intermediate product isverified by a standard ICP test.3) 2^(nd) blending: the lithium deficient sintered precursor is blendedwith LiOH.H₂O in order to correct the Li stoichiometry in theintermediate product to the final target composition of Li_(1.017)(Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2))_(0.983) O₂ (NMC 622). Theblending is performed in a Henschel Mixer® for 30 mins.4) 2^(nd) sintering: the blends (from step 3) are sintered at 850° C.for 10 hours under dry air atmosphere in a tray based furnace. The dryair is continuously pumped into the equipment at a flow rate of 40 L/hr.5) Post treatment: after the 2^(nd) sintering, the sintered cake iscrushed, classified and sieved so as to obtain a non-agglomerated NMCpowder.

MANUFACTURING EXAMPLE 3 Example According to the Invention

This invention discloses a process to obtain high Ni-excess NMC with alow soluble base content by a double firing method and is illustrated inFIG. 1. The double firing includes a 1^(st) sintering (F1) whichdelivers a lithium deficient intermediate NMC product (P1), and a secondsintering delivering the final lithium metal oxide (P2). First, atransition metal precursor (M1) is mixed with a lithium source (L1).Transition metal precursors are selected from hydroxides, oxyhydroxides,carbonates, mixed oxide etc. Preferable Ni, Mn and Co are present andwell mixed at atomic scale. The lithium source is selected from lithiumhydroxide, lithium hydroxide hydrate or lithium oxide. The lithiumsource is essentially free of Li₂CO₃. Additionally additives (A1) likeAl₂O₃, MgO etc. may be added for obtaining a doped final material.Additives can be oxides, hydroxides, carbonates, sulfates etc. A 1^(st)sintering step (F1)—which is the pre-firing—is applied and provides alithium deficient intermediate NMC product (P1). After that theintermediate NMC product pre-fired product is blended with an additionallithium source (L2). The lithium source is selected from lithiumhydroxide, lithium hydroxide hydrate or lithium oxide. The lithiumsource is essentially free of Li₂CO₃. Additionally additives or dopants(A2) may be added. A 2^(nd) sintering step (F2)—which is the firing—isapplied and provides the final lithium transition metal oxide (P2).

For producing an NMC at high throughput with high quality on alarge-scale, the 1^(st) sintering is carried out in a rotary furnace orkiln. This improves the non-uniform Li:M composition effect, and alsoallows very high throughput because the heat transfer issue is resolvedin a rotary furnace. A product in a rotary furnace has a short residencetime. Typical residence times in the heated zone of a rotary furnace areat least 20 min and typically less than 3 hr. If the residence time istoo short the reaction is not complete. If the residence time is toolong, the throughput is insufficient. A typical temperature range forthe 1^(st) sintering is 650° C. to 850° C. If the temperature is too lowthe reaction is not complete. If the temperature is too high the metalof the tube tends to react with the lithiated NMC.

The intermediate NMC after the 1^(st) sintering is lithium deficient.Whereas the final lithium transition metal oxide has a Li:M′stoichiometric ratio near to unity, the intermediate NMC has a targetLi:M′ stoichiometric range of 0.65-0.94. The lithium deficiency allowsto finish the lithiation reaction during the short residence time withinthe heated zone of the rotary furnace. Lithium deficiency also reducesmetal corrosion as it reduces the reactivity of lithium with the metaltube. Thus the lithium deficiency of the intermediate NMC is critical toachieve high throughput during the 1^(st) sintering. The 1^(st)sintering in the rotary furnace uses an oxidizing gas, preferably air.However, if high Ni excess cathodes are the target, oxygen might be apreferred choice. Generally lithium deficiency allows to reduce thecarbonate impurity in the intermediate NMC. Whereas a fully lithiatedNMC will take up CO₂ from the air to form Li₂CO₃, a lithium deficientNMC has a stronger tendency not to react with the CO₂ in the air or evento decompose the Li₂CO₃ impurity during the 1^(st) sintering. Thus thelithium deficiency is critically linked to the use of air during the1^(st) sintering.

During the 2^(nd) sintering the lithium deficient NMC is sintered toachieve the final lithium transition metal oxide. First, the lithiumdeficient NMC is blended with a source of lithium to obtain the finalLi:M stoichiometric target value. Then the mixture is fired to obtain awell sintered product. The 2^(nd) sintering is typically done usingceramic trays and a suitable furnace. Furnaces can be large chamberfurnaces where stacks of trays are fired. More suitable are rollerhearth kilns where trays with product are carried across the furnace.Alternatively pusher kilns can be applied where carts with stacks oftrays are carried across the furnace. Other furnace designs can beapplied as well. Less desired are rotary furnaces. The rotary furnacehas a short residence time which might not allow to achieve awell-sintered high quality product. Another issue linked to rotaryfurnaces are the sintering temperatures. The sintering temperaturesneeded during the 2^(nd) sintering are relatively high, and still highertemperatures would be required if the residence time is short. Undersuch conditions metal tubes tend to react with lithiated product andmetal corrosion is observed. After the 2^(nd) sintering NMC powder isachieved having a low soluble base and on a large-scale.

The invention observes that the properties of the lithium deficientsintered product strongly influence the performance of the final productin batteries. Particularly, the soluble base content of the finalproduct is strongly related to the conditions during the 1^(st)sintering. The 1^(st) sintering temperature, furnace type, tray loadingand the ratio of lithium to the mixed transition metal source can bechosen appropriately to obtain a final product of high quality and witha high throughput. For example, if the ratio of lithium to metal sourceis too high, the reaction between the mixed transition metal source andthe lithium source doesn't finish and results in unreacted and moltenlithium sources, which can attack the inner wall of a furnace. At thesame time, it causes agglomeration of NMC material and uptake of CO₂from the air, which induces a poor cycling performance in batteries. Ifon the contrary the ratio of lithium to metal is too low, a large amountof lithium is required to adjust the Li stoichiometry in theintermediate product to the final target composition during the 2^(nd)sintering. Because the lithium source causes an excess vapor evolutionduring heating, the final product has an inhomogeneous chemicalcomposition. Therefore, the ratio of lithium to metal during the 1^(st)firing must be optimized to produce high quality NMC product. Theproperties of the NMC product using the lithium deficient sinteredprecursor will be checked by various parameters: the crystalline size ofthe product after the 1^(st) sintering step, the Li₂CO₃ content, and thecycling performance of the final product.

Furthermore, to reduce the Li₂CO₃ content in the final NMC material,LiOH.H₂O as lithium source can be used in both of the 1^(st) and 2^(nd)sintering steps. The double sintering using a combination of rotary andtray based furnace provides the large-scale production of NMC productwith low soluble base content at low cost. Consequently, the throughputis much higher compared to the direct sintering method described inManufacturing Example 1 and the double sintering method of ManufacturingExample 2 based only on conveyor furnaces. Thus, the use of the lithiumdeficient sintered precursor and applying the double firing method inthis invention is less expensive and a more efficient manufacturing wayfor Ni-excess NMC.

In this invention, to reduce the soluble base in the final product, themixed transition metal source, which is used during the 1^(st) blendingmay be roasted. It might be useful to reduce the amount of vapor whichevolves during the first firing. If a mixed hydroxide M′(OH)₂ precursoris roasted at 250° C., for example, then a M′OOH type precursor isachieved which evolves less H₂O. If in another example the precursor isroasted at 375° C. a mixed oxide is predominantly achieved, which doesnot evolve substantial amounts of vapor. The roasting can be performedin air, oxygen, nitrogen or a dynamic vacuum. The roasted mixedtransition metal source is blended with LiOH.H₂O and then sintered forthe formation of a lithium deficient sintered precursor. When using theroasted mixed transition metal oxide, a final NMC material with lowsoluble base content is obtained in a large-scale production.

During the 1^(st) sintering, the firing time may also be optimized toguarantee the reaction processing forward to the maximum extent. In anembodiment, the total sintering time including heating and cooling isset in the range of 12 to 20 hours for the large-scale production ofNMC. After the 1^(st) sintering, a lithium deficient sintered precursoris obtained. The precursor has a low content of Li₂CO₃ impurity. In anembodiment, it is determined by pH titration that the Li₂CO₃ content is<0.40 wt %, preferably <0.20 wt %. The intermediate product is a singlephase lithium transition metal oxide having an ordered or disorderedrock salt crystal structure. The composition is believed to beLi_(1−x)M_(1+x)O₂. In an embodiment the Li:M stoichiometric ratio is0.48 to 0.94, preferably 0.7 to 0.9. The metal composition isLi_(1−a)((Ni_(z)(Ni_(1/2) Mn_(1/2))_(y)Co_(x))_(1−k) A_(k))_(1+a) O₂,wherein x+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and0.03≤a≤0.35. The precursor has a crystalline size L expressed in nm thatis dependent on the Ni excess content z, with 15≤L≤36. The dopant may beeither one or more of Ti, Mg, W, Zr, Cr, V and Al. These dopants cancontribute to an improvement of the performance and safety of a batterycontaining the final cathode material made from the precursor.

In this invention, the double sintering method using a rotary furnacefor the 1^(st) sintering increases the throughput of NMC, and uses muchless space than a conveyor furnace. As the investment cost roughlycorrelates with the required space, the investment for the rotaryfurnace is much less than that of the conveyor furnace. Moreover, theprecursor and lithium source can be filled easily and unloaded in therotary furnace, whereas the conveyor furnace usually requires a complextray filling equipment. Therefore, to use the double sintering methodbased on the rotary furnace for the 1^(st) sintering enhances thethroughput of the final NMC product and reduces the investment costdramatically.

Surface Coating Example 1

Referring back to FIG. 1, an aluminum coated NMC is obtained by blendingand sintering a final lithium transition metal oxide powder (F3) and forexample an aluminum source (A3). The aluminum source in this step can bea metal oxide (Al₂O₃) that may be combined with a compound selected fromthe group consisting of TiO₂, MgO, WO₃, ZrO₂, Cr₂O₃, V₂O₅ and mixturesthereof. The preferred source of aluminum is a nanometric aluminapowder, for example fumed alumina. In the heating step, the mixture isheated at about 750° C. The sintering time is preferably at least 3hours, more preferably at least 5 hours. The final product may have anAl content more than 0.3 mol % but less than 3.0 mol %.

Surface Coating Example 2

Referring back to FIG. 1, an aluminum and fluorine coated NMC isobtained by blending and subsequent sintering (F3) using an aluminumsource (as described before) and a fluorine-containing polymer (A3). Atypical example for such a polymer is a PVDF homopolymer or PVDFcopolymer (such as HYLAR® or SOLEF® PVDF, both from Solvay SA, Belgium).Another known PVDF based copolymer is for example a PVDF-HFP(hexa-fluoro propylene). Such polymers are often known under the name“Kynar®”. Teflon—or PTFE—could also be used as polymer. For thesintering step, the sintering temperature of the mixture is at least250° C., preferably at least 350° C. The sintering time here ispreferably at least 3 hours, more preferably at least 5 hours. In thesintering step, the crystalline structure of the fumed alumina ismaintained during the coating process and is found in the coating layersurrounding the lithium metal oxide core. Also, the fluorine-containingpolymer is completely decomposed and lithium fluoride is formed, whichis found in the surface layer of the particles. The obtained surfacelayer has the following function: the thin layer comprising LiF replacesthe reactive surface base layer, thus reducing the base contentpractically to zero at the core's surface, and improving the overallsafety.

Surface Coating Example 3

Referring back to FIG. 1, an aluminum and sulfate coated NMC is obtainedby blending and subsequent sintering (F3) using an aluminum source (asdescribed before) and a sulfur-containing source (A3). Thesulfur-containing source in this step can be either one of Li₂S₂O₈,H₂S₂O₈ and Na₂S₂O₈. In the sintering step, the blend is heated between300 and 500° C.—and preferably 375° C.—under air. The sintering time isat least 3 hours, and preferably at least 5 hours. The final productcontains a coating of a sulfate and aluminum, which improves batteryperformance by decomposition of soluble surface base compounds.

Description of Analysis Methods:

A) pH Titration Test

The soluble base content is a material surface property that can bequantitatively measured by the analysis of reaction products between thesurface and water, as is explained in WO2012-107313. If powder isimmersed in water a surface reaction occurs. During the reaction, the pHof the water increases (as basic compounds dissolve) and the basecontent is quantified by a pH titration. The result of the titration isthe “soluble base content” (SBC). The content of soluble base can bemeasured as follows: 2.5 g of powder is immersed in 100 ml of deionizedwater and stirred for 10 mins in a sealed glass flask. After stirring todissolve the base, the suspension of powder in water is filtered to geta clear solution. Then 90 ml of the clear solution is titrated bylogging the pH profile during addition of 0.1 M HCl at a rate of 0.5ml/min until the pH reaches 3 under stirring. A reference voltageprofile is obtained by titrating suitable mixtures of LiOH and Li₂CO₃dissolved in low concentration in DI water. In almost all cases twodistinct plateaus are observed. The upper plateau with endpoint γ1 (inml) between pH 8˜9 is the equilibrium OH⁻/H₂O followed by theequilibrium CO₃ ²⁻/HCO₃ ⁻, the lower plateau with endpoint γ2 (in ml)between pH 4˜6 is HCO³⁻/H₂CO₃. The inflection point between the firstand second plateau γ1 as well as the inflection point after the secondplateau γ2 are obtained from the corresponding minima of the derivatived_(pH)/d_(Vol) of the pH profile. The second inflection point generallyis near to pH 4.7. Results are then expressed in LiOH and Li₂CO₃ weightpercent as follows:

${{{Li}_{2}{CO}_{3}{wt}\mspace{14mu}\%} = {\frac{73.8909}{1000} \times \left( {\gamma_{2} - \gamma_{1}} \right)}};$${{LiOH}\mspace{14mu}{wt}\mspace{14mu}\%} = {\frac{23.9483}{1000} \times {\left( {{2 \times \gamma_{1}} - \gamma_{2}} \right).}}$B) Valence State Titration Test

In this invention, the average valence state of products is determinedby auto-titration using a Mettler Toledo Autotitrator DL70ES. Thetitrant is freshly made potassium dichromate water solution with aconcentration of 0.01493 mol/L. Prior to titrant preparation, K₂Cr₂O₇ isdried at 104° C. for at 2 hours. The reducing agent is freshly preparedferrous ammonium sulfate water solution. First, 156.85 g Fe(NH₄)₂(SO₄)₂is weighed in a beaker. About 250 ml of nano-pure water and about 5 mlof 1:1 sulfuric acid are added. Heat may be applied to speed up thedissolution process. Afterwards, the solution is transferred into a 1 lvolumetric flask and diluted to marked volume at 20° C. before use. 0.5g to 3.0 g of NMC precursor sample is weighed into a digestion tube. 20ml of freshly made Fe(NH₄)₂(SO₄)₂ solution and 10 ml of concentrated HClare added into the digestion tube. Heat may be applied here for completedigestion of the sample. The solution is fully transferred into a 100 mlvolumetric flask and diluted to the volume mark at 20° C. Afterwards, 10ml of this solution is pipetted into a titration cup together with 5 mlof 1:1 HCl and 40 ml of nano-pure water to the cup. The NMC precursorsample solution is now ready for valence titration. The same procedureis repeated to prepare a reference sample solution (without NMCprecursor) using exactly the same amount of Fe(NH₄)₂(SO₄)₂ solution,concentrated HCL, 1:1 HCl, nano-pure water under similar conditions.Using the above prepared K₂Cr₂O₇ solution, both NMC precursor samplesolution and reference sample solution are titrated by using MettlerToledo Autotitrator DL70ES. The volume of titrant consumed are recordedfor each titration. The difference in volume is used for valence statecalculation.

C) Karl Fischer Titration Test

The typical moisture content of precursor samples after drying is below1 wt %, is determined by Karl Fischer at 250° C. KF 34739-Coulomat AGOven is used as a reagent and added until the water in the samples isremoved.

D) X-Ray Diffraction Test

In this invention, the crystallinity of NMC samples is evaluated bydetermining the crystalline size and lattice strain from the X-raydiffraction patterns. The diffraction patterns are collected with aRigaku X-Ray Diffractometer (D/MAX-2200/PC). The scan speed is set atcontinuous scanning at 1 degree per minute. The step-size is 0.02 degreeScans are performed between 15 and 85 degree.

The crystalline size, as a derivation from perfect crystallinity, leadsto a broadening of a diffraction peak. It is the same case for strain,which is defined as a deformation of unit cell divided by its length,represented by Δd/d. The non-uniform lattice strain can cause thesystematic shifts of atoms and lead to a peak broadening. Thus, throughthe analysis of the width of individual diffraction peaks, thecrystalline size and lattice strain could be obtained.

In “Acta Metallurgica, 1, 22-31 (1953)”, Williamson and Hall proposed amethod to extract the information on crystalline size and strain fromthe integral width of diffraction peaks. This method is based on theapproximate relationship between Bragg angle (θ) and peak broadeningarising from crystalline size and lattice strain, with the followingformula:

${\beta cos\theta} = {{C\;{\epsilon sin\theta}} + \frac{K\;\lambda}{L}}$where β represents the integral width of peak, ϵ is the lattice strain,L is the crystalline size, λ is the radiation wavelength, and C and Kare constants, often taken as 4 and 0.9, respectively. By looking at theproduct of integral width (β) and cos θ as a function of sin θ, thelattice strain and crystalline size can be estimated from the slope andintercept of a fitting line for this formula, respectively. The integralwidth (β) is the width of a rectangle having the same height (maximumintensity) and area (integrated intensity) of the selected diffractionpeak. The area can be approximately integrated by a trapezoidal rule,and the height can be easily obtained from raw data of the diffractionpattern, thus it is feasible to estimate the integral width of eachdiffraction peak and further determine the crystalline size and latticestain by this Williamson-Hall (W-H) method.

In this invention, the (003) and (104) peaks at 17°-20° and 43°-45.5°respectively are chosen to calculate the crystalline size and strain.The integral width and Bragg angle of diffraction peak (003) arerepresented by β₁ and θ₁, while the integral width and Bragg angle ofdiffraction peak (104) are represented by β₂ and θ₂. The crystallinesize L and lattice strain ϵ can be obtained from the intercept andslope, by the following formulas:

$\begin{matrix}{L = \frac{K\;\lambda}{\gamma_{2} - {\frac{\gamma_{2} - \gamma_{1}}{x_{2} - x_{1}} \times x_{2}}}} & {ɛ = \frac{\frac{\gamma_{2} - \gamma_{1}}{x_{2} - x_{1}}}{C}}\end{matrix}$Where the γ₂ is defined as the product of β₂ and cos θ₂, γ₁ is definedas the product of β₁ and cos θ₁. The x₂ and x₁ are the value of sin θ₂and sin θ₁ respectively.

It is known that the structural model of Li_(1−a)((Ni_(z) (Ni_(1/2)Mn_(1/2))_(y) Co_(x))_(1−k) A_(k))_(1+a) O₂ is the α-NaFeO₂ structure(space group R-3m, no. 166) with Li in 3a sites, Ni, Co, and Mn randomlyplaced on 3b sites, and oxygen atoms on 6c sites (in general an NMCcompound can be represented as [Li]_(3a)[Ni_(x)Co_(y)Mn_(z)]_(3b)[O₂]_(6c)). The current invention howeverobserves that the lithium deficient sintered precursor has a phenomenonof cation mixing, meaning that there is a high amount of Ni on Li 3asites (being the sites within the layers predominantly filled by Liatoms). This differentiates our lithium deficient sintered precursorfrom the common lithium deficient material obtained duringcharge/discharge. The latter basically has little cation mixing.Generally, the degree of Li/M disorder can be roughly estimated by theintensity ratio of peak (003) (referred to as I003) to I104 (=intensityof peak (104)), as indicated in “J. Electrochem. Soc. 140 (1993) 1862”.A large ratio of I003 to I104 means a low degree of Li/M disorder. Asystematic study on cation mixing was described by Jeff Dahn in SolidState Ionics 44 (1990) 87-97. U.S. Pat. No. 6,660,432 B2 gives anextended application of this method to evaluate the degree of Li/Mdisorder on Li-in excess transition metal oxide material. The idea ofthis method originates from the fact that the intensity I101 of peak(101) at 35°-37.2° is rapidly attenuated while the combinationalintensity of peaks (006) and peak (102) at 37.2° to 39.2° (I006 & I102)are enhanced when Ni atoms occupy “Li sites”. Thus, a factor of R isintroduced, which represents the ratio of I006 & I102 to I101. In Dahn'spaper, it is demonstrated that the R factor increases rapidly as xdecreases in Li_(x)Ni_(2−x)O₂ material, where 1−x refers to the degreeof cation mixing. A formula was deducted to express the relationshipbetween R and x as follows:

$R = {\frac{4}{3}\frac{\left( {1.6 - x} \right)^{2}}{x^{2}}}$

So the degree of cation mixing (1−x) is equivalent to R, and can bedetermined from the R value according to the formula.

In this invention, the two methods here above are used to evaluate thedegree of cation mixing of the lithium deficient sintered precursors andthe final products based on these precursors. The ratio I003/I104 andthe value of R will be discussed below. It is observed that the degreeof cation mixing is higher in a lithium deficient sintered precursor bycontrast to the final product. Note that to calculate the ratioI003/I104 and the value of R, integrated XRD peaks are used.

E) Coin Cell Test

Coin cells are assembled in a glovebox which is filled with an inert gas(argon). A separator (SK Innovation) is located between the positiveelectrode and a piece of lithium foil used as negative electrode.1MLiPF6 in EC/DMC (1:2) is used as electrolyte, dropped betweenseparator and electrodes. Each cell is cycled at 25° C. usingToscat-3100 computer-controlled galvanostatic cycling stations (fromToyo). The coin cell testing schedule used to evaluate NMC samples isdetailed in Table 1. The schedules use a 1C current definition of 160mA/g and comprise three parts as follows:

Part I is the evaluation of rate performance at 0.1C, 0.2C, 0.5C, 1C, 2Cand 3C in the 4.3˜3.0V/Li metal window range. With the exception of the1^(st) cycle where the initial charge capacity CQ1 and dischargecapacity DQ1 are measured in constant current mode (CC), all subsequentcycles feature a constant current-constant voltage during the chargewith an end current criterion of 0.05C. A rest time of 30 minutes forthe first cycle and 10 minutes for all subsequent cycles is allowedbetween each charge and discharge. The irreversible capacity Q_(irr). isexpressed in % as:

$Q_{{Irr}.} = {\frac{\left( {{{CQ}\; 1} - {{DQ}\; 1}} \right)}{{CQ}\; 1} \times 100(\%)}$

The rate performance at 0.2C, 0.5C, 1C, 2C and 3C is expressed as theratio between the retained discharge capacity DQn, with n=2, 3, 4, 5 and6 for respectively nC=0.2C, 0.5C, 1C, 2C and 3C as follows:

${{nC} - {rate}} = {\frac{DQn}{{DQ}\; 1} \times 100(\%)}$

For example,

${{3C} - {{rate}\left( {{in}\mspace{14mu}\%} \right)}} = {\frac{{DQ}\; 6}{{DQ}\; 1} \times 100}$

Part II is the evaluation of cycle life at 1C. The charge cutoff voltageis set as 4.5V/Li metal. The discharge capacity at 4.5V/Li metal ismeasured at 0.1C at cycles 7 and 34 and 1C at cycles 8 and 35. Capacityfadings at 0.1C and 1C are calculated as follows and are expressed in %per 100 cycles:

${0.1C\mspace{14mu}{{QFad}.}} = {\left( {1 - \frac{{DQ}\; 34}{{DQ}\; 7}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$${1C\mspace{14mu}{{QFad}.}} = {\left( {1 - \frac{{DQ}\; 35}{{DQ}\; 8}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$

Energy fadings at 0.1C and 1C are calculated as follows and areexpressed in % per 100 cycles. Vn is the average voltage at cycle n.

${0.1C\mspace{14mu}{{EFad}.}} = {\left( {1 - \frac{{DQ}\; 34 \times \overset{\_}{V\; 34}}{{DQ}\; 7 \times \overset{\_}{V\; 7}}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$${1C\mspace{14mu}{{EFad}.}} = {\left( {1 - \frac{{DQ}\; 35 \times \overset{\_}{V\; 35}}{{DQ}\; 8 \times \overset{\_}{V\; 8}}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$

Part III is an accelerated cycle life experiment using 1C-rate for thecharge and 1C rate for the discharge between 4.5 and 3.0V/Li metal.Capacity and energy fading are calculated as follows:

${1C\text{/}1C\mspace{14mu}{{QFad}.}} = {\left( {1 - \frac{{DQ}\; 60}{{DQ}\; 36}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$${1C\text{/}1C\mspace{11mu}{{EFad}.}} = {\left( {1 - \frac{{DQ}\; 60 \times \overset{\_}{V\; 60}}{{DQ}\; 36 \times 36}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$

TABLE 1 coin cell testing procedure Charge Discharge Cycle End Rest V/LiEnd Rest V/Li Type No C Rate current (min) metal (V) C Rate current(min) metal (V) Part I 1 0.10 — 30 4.3 0.10 — 30 3.0 2 0.25 0.05 C 104.3 0.20 — 10 3.0 3 0.25 0.05 C 10 4.3 0.50 — 10 3.0 4 0.25 0.05 C 104.3 1.00 — 10 3.0 5 0.25 0.05 C 10 4.3 2.00 — 10 3.0 6 0.25 0.05 C 104.3 3.00 — 10 3.0 Part II 7 0.25  0.1 C 10 4.5 0.10 — 10 3.0 8 0.25  0.1C 10 4.5 1.00 — 10 3.0  9~33 0.50  0.1 C 10 4.5 1.00 — 10 3.0 34  0.25 0.1 C 10 4.5 0.10 — 10 3.0 35  0.25  0.1 C 10 4.5 1.00 — 10 3.0 PartIII 36~60 1.00 — 10 4.5 1.00 — 10 3.0

The following examples illustrate the present invention in more detail.

Explanatory Example 1 NMC Samples Prepared Using Direct and DoubleSintering

An NMC powder is prepared according to the above-mentioned“Manufacturing Example 1” with Li₂CO₃ as Li source. This sample islabelled NMC P1.1. Also, NMC powder is prepared as in “ManufacturingExample 2”, based on a conveyor furnace for the 1st and 2nd sintering,and is labelled NMC P1.2. Finally, NMC powder is prepared by the“Manufacturing Example 2”, but using a rotary furnace during the 1stsintering and is labelled NMC P1.3. In all the Examples of thisinvention, mixed nickel-manganese-cobalt hydroxides (M′-hydroxides,where M′=Ni0.6Mn0.2Co0.2 unless otherwise mentioned) are used asprecursors, where M′-hydroxide is prepared by a co-precipitation inlarge scale continuous stirred tank reactor (CSTR) with mixednickel-manganese cobalt sulfates, sodium hydroxide, and ammonia. In thiscase, the general formula of the M′-hydroxide is(Ni_(0.4)(Ni_(1/2)Mn_(1/2))_(0.4)Co_(0.2))(O)_(v)(OH)_(w), with 0≤v≤1and v+w=2.

FIG. 2 presents the pH titration results of these NMC materials, wherethe weight percentage of lithium carbonate in the final NMC samples isplotted. The prepared powders have a large distinction in base content.NMC P1.2 sample has less lithium carbonate than NMC P1.1 because of theused double sintering method combined with a Li deficient precursor. Asmentioned above, although a rotary furnace for the 1^(st) sintering stepduring the double sintering method is suitable for a large-scaleproduction of NMC, NMC P1.3 sample has a higher residual lithiumcarbonate content than the other two samples. Therefore, it is useful toinvestigate if LiOH.H₂O instead of Li₂CO₃ as a lithium source for the1^(st) sintering can reduce the soluble base content by its lowerthermodynamic stability during the preparation of NMC, leading to goodelectrochemical performance. The following examples will illustrate thisin detail.

Explanatory Example 2 NMC Samples Prepared Using Pretreated TransitionMetal Source and Direct Sintering

An NMC powder is prepared based on a direct sintering method using apristine mixed transition metal hydroxide type source and LiOH.H₂O. Thissample was prepared on small scale and is labelled NMC P2.1. Another NMCpowder is prepared using a pretreated mixed transition metal source on alarge-scale using direct sintering, and is labelled NMC P2.2. For thepretreatment of the metal source, a mixed transition metal oxide isheated at 150° C. under N₂ atmosphere in an oven. Finally, an NMC powderis manufactured using the same step in NMC P2.2, except that the heatingtemperature of the pretreated metal source is 250° C. It is labelled NMCP2.3. To investigate also the properties of pretreated transition metalsources themselves, we labelled the transition metal sourcesrespectively NMC P2.1a (not heated), NMC P2.2a (heated at 150° C.), andNMC P2.3a (heated at 250° C.). FIG. 3 indicates the XRD patterns of thepretreated transition metal samples. After heating, the XRD peaks of thesources shifted, which means the oxidation state changed. In Table 2,the properties of the pretreated transition metal samples are shown indetail. The values of oxidation state are calculated using a ‘Valencestate titration’ method.

TABLE 2 Properties of pretreated transition metal samples SampleOxidation state (n⁺) H₂O (%) Mass loss (%) NMC P2.1a 2.17 17.413 — NMCP2.2a 2.53 12.605 0.89% NMC P2.3a 2.71 2.805 9.49%

NMC P2.1a has an oxidation state of 2.17, corresponding to the formula(Ni_(0.4)(Ni_(1/2)Mn_(1/2))_(0.4)Co_(0.2))(O)_(0.17)(OH)_(1.83). Afterheating at 150° C., the oxidation state of NMC P2.2a is 2.53, and theformula becomes(Ni_(0.4)(Ni_(1/2)Mn_(1/2))_(0.4)Co_(0.2))(O)_(0.53)(OH)_(1.47). Theoxidation state of NMC P2.3a is 2.71, and the formula becomes(Ni_(0.4)(Ni_(1/2)Mn_(1/2))_(0.4)Co_(0.2))(O)_(0.71)(OH)_(1.29).

By increasing the pretreatment temperature, the transition metal sourcehas a higher oxidation state than pristine source. Also, to investigatethe moisture content of the pretreated precursors, their H₂O % contentis analyzed after being heated at 300° C. NMC P2.3a has the lowestmoisture content. By increasing the heating temperature, the mass lossof the precursor increases, compared to the initial weight. Table 3summarizes the pH titration and coin cell results of NMC P2.1, P2.2, andP2.3.

TABLE 3 Performance of Explanatory Example 2 DQ0.1 C 0.1 C QFad. 1 CQFad. Li₂CO₃ Sample (mAh/g) (%/100) (%/100) (wt %) NMC P2.1 178.4 1.55.9 0.138 NMC P2.2 174.3 4.4 10.2 0.306 NMC P2.3 175.0 3.2 9.0 0.229

The weight percentage of lithium carbonate in the final NMC P2.2 sampleis determined at 0.3061 wt %, which is a high amount compared to thecontent of NMC P2.1. Because the NMC product is prepared by directsintering on a large-scale, it contains large amounts of lithiumcarbonate. Thus, by using a roasted transition metal source with low H₂Ocontent the amount of soluble base may be reduced, as is shown for theNMC P2.3 sample.

The presence of high soluble base and Li₂CO₃ contents in the final NMCmaterial generally deteriorates the cycling performance. The coin celltest evaluates the cycle stability of NMC P2.1, NMC P2.2, and NMC P2.3samples based on the capacity fade at 0.1 C and 1C. It shows that theNMC P2.1 sample has 0.015% loss of discharge capacity per cycle at 0.1 Cafter 25 cycles and 0.059% loss for 1C. The NMC P2.2 sample has 0.044%loss of discharge capacity per cycle at 0.1C after 25 cycles and 0.102%loss for 1C. When the heated mixed transition metal oxide at 150° C. isused as a metal source for large-scale production, it shows the worsecycling performance due to its high lithium carbonate content. Moreover,in case of NMC P2.3 prepared using the pretreated metal source at 250°C., it has a better cycle stability than NMC P2.2. Therefore, for thelarge-scale production, it is possible to produce NMC with enhancedbattery performance by using a high-temperature pretreated transitionmetal source.

EXAMPLE 1 NMC Samples Prepared Using Double Sintering

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ withM′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2) is manufactured from alithium deficient sintered precursor with ratio of Li:M=0.921 throughthe above-mentioned “Manufacturing Example 3”: steps F1 and F2.

In the 1^(st) sintering step, LiOH.H₂O is used as lithium precursor toproduce the Li deficient precursor in a rotary furnace. The mixture oftransition metal source and lithium precursor is sintered at 820° C. for2 hours of residence time and 1.316 rpm of rotation speed under dry airwith a rate of 2 m³/kg. The 2^(nd) sintering is conducted at 860° C. for10 hours under dry air atmosphere in a tray based furnace. The dry airis continuously pumped into the equipment at a flow rate of 40 L/hr. Theabove prepared lithium deficient sintered precursor after the 1^(st)sintering is labelled NMC E1p, and the final NMC sample after 2^(nd)sintering is labelled NMC E1.

FIG. 4 shows the XRD patterns of NMC E1p and NMC E1. The Bragg peaks(003), (101), (104) and doublet peak (006, 102) in that order are thehighest in the patterns. Based on the intensity of these peaks, Table 4summarizes the ratio of I003/I104 and R factor of the NMC E1p and E1samples.

TABLE 4 I003/I104 ratio and R factor of Example 1 Sample I003/I104 Rfactor NMC E1p 0.87 0.60 NMC E1 1.02 0.40

As described above, the ratio of I003/I104 reflects the degree of Li totransition metal disorder. A large value of I003/I104 indicates a smalldegree of distortion. The precursor sample NMC E1p has a small I003/I104ratio, which means there is more cation mixing in NMC E1p and more Ni onthe Li sites. The same observation can be made when comparing the Rfactor. The lithium deficient sintered precursor has a higher R factorby contrast to the final product. As discussed in Dahn's paper mentionedabove, a high R factor means a high disordering of Li and transitionmetals. Thus, the higher value of R in NMC E1p confirms that there is ahigher percentage of Ni on Li sites in the lithium deficient sinteredprecursor. Table 5 summarizes the electrochemical performance andsoluble base content of NMC E1.

TABLE 5 Performance of Example 1 0.1 C QFad. 1 C QFad. Sample DQ0.1 C(mAh/g) (%/100) (%/100) Li₂CO₃ (wt %) NMC E1 179.5 0.8 5.4 0.206

The NMC E1 sample contains much less weight percentage of lithiumcarbonate than NMC P1.1, P1.2, and P1.3. It shows that there is 0.008%loss of discharge capacity per cycle at 0.1C after 25 cycles and 0.054%loss for 1C.

EXAMPLE 2 NMC Samples Prepared Using Roasted Transition Metal Source andDouble Sintering

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ with M′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2) is prepared using the steps in Example1, except that the lithium deficient sintered precursor has a ratio ofLi:M=0.885 and that the mixed transition metal source is used afterroasting at 250° C. under N₂ atmosphere in an oven for 24 hr. In the1^(st) sintering step, the mixture of transition metal source andLiOH.H₂O is sintered at 820° C. for 2 hours of residence time and 0.628rpm of rotation speed under dry air with flow rate of 1.67 m³/kg in arotary furnace. The 2^(nd) sintering is conducted at 865° C. for 10hours under dry air atmosphere in a tray based furnace. The dry air iscontinuously pumped into the equipment at a flow rate of 40 L/hr. Theabove prepared lithium deficient sintered precursor after 1^(st)sintering is labelled NMC E2p, and the final NMC sample after 2^(nd)sintering is labelled NMC E2. FIG. 5 shows the XRD patterns of NMC E2pand NMC E2. The Bragg peaks (003), (101), (104) and doublet peak (006,102) are clearly visible. Based on the intensity of these peaks, Table 6summarizes the ratio of I003/I104 and R factor of the NMC E2p and E2samples.

TABLE 6 I003/I104 ratio and R factor of Example 2 Sample I003/I104 Rfactor NMC E2p 0.81 0.66 NMC E2 1.00 0.41

Looking at the ratio I003/I104, it can be concluded that there is morecation mixing in NMC E2p and more Ni on the Li sites. The sameobservation can be made when comparing the R factor. The higher value ofR in NMC E2p confirms that there is a higher percentage of Ni on Lisites in the lithium deficient sintered precursor. Table 7 summarizesthe electrochemical performance and soluble base content of NMC E2.

TABLE 7 Performance of Example 2 DQ0.1 C 0.1 C QFad. 1 C QFad. Li₂CO₃Sample (mAh/g) (%/100) (%/100) (wt %) NMC E2 176.7 0.2 5.4 0.196

The NMC E2 sample shows less weight percentage of lithium carbonate thanNMC P1.1, P1.2, and P1.3. It shows that there is 0.002% loss ofdischarge capacity per cycle at 0.1C after 25 cycles and 0.054% loss for1C. Therefore, the double sintering method using the roasted transitionmetal source enhances the cycle properties of the NMC product.

EXAMPLE 3 NMC Samples Prepared Using the Intermediate Product with LowRatio of Li:M

An NMC powder having the formula Li_(1.017)M′_(0.983)O₂ with M′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2) is obtained according to the steps inExample 2 (including the preroasting step), except that the lithiumdeficient sintered precursor has a ratio of Li:M=0.718. In the 1^(st)sintering step, the mixture of transition metal source and LiOH.H₂O issintered at 820° C. for 2 hours of residence time and 0.628 rpm ofrotation speed under dry air with flow rate of 1.67 m³/kg in a rotaryfurnace. The 2^(nd) sintering is conducted at 855° C. for 10 hours underdry air atmosphere in a tray based furnace. The dry air is continuouslypumped into the equipment at a flow rate of 40 L/hr. The above preparedlithium deficient sintered precursor after the 1st sintering is labelledNMC E3p, and the final NMC sample after the 2nd sintering is labelledNMC E3.

EXAMPLE 4 NMC Samples Prepared Using the Intermediate Product with LowRatio of Li:M at Low Temperature

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ with M′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2) is prepared according to the steps inExample 2 (including the preroasting step), except that the lithiumdeficient sintered precursor has a ratio of Li:M=0.723, as it isprepared at a low 1^(st) sintering temperature of 720° C. In the 1^(st)sintering step, the mixture of transition metal source and LiOH.H₂O issintered at 720° C. for 2 hours of residence time and 0.628 rpm ofrotation speed under dry air with flow rate of 1.67 m³/kg in a rotaryfurnace. The 2^(nd) sintering is conducted at 845° C. for 10 hours underdry air atmosphere in a tray based furnace. The dry air is continuouslypumped into the equipment at a flow rate of 40 L/hr. The above preparedlithium deficient sintered precursor after 1st sintering is labelled NMCE4p, and the final NMC sample after 2^(nd) sintering is labelled NMC E4.

FIG. 6 shows the X-ray diffraction patterns of NMC E3p and NMC E4p,where the intermediate means the Li-deficient NMC powder obtained afterthe 1^(st) sintering. The XRD patterns disclose single-phase NMC powderswithout obvious impurities. In the Figure, the (003) and (104)diffraction peaks are used to calculate the crystalline size L andlattice strain with the W-H method. FIG. 7 shows the coin cell resultsof the NMC E3 and NMC E4 samples, where the square symbol is for NMC E3and the circle symbol is for NMC E4. It can be observed that the NMC E4has a similar but slightly better cycling stability than NMC E3, andTable 8 summarizes the electrochemical performance and soluble basecontent of NMC E3 and E4.

TABLE 8 Performance of Example 3 and Example 4 1 C *Size (nm) by DQ0.1 C0.1 C QFad. QFad. Li₂CO₃ Sample W-H (mAh/g) (%/100) (%/100) (wt %) NMCE3 33.8 179.0 4.0 8.7 0.245 NMC E4 31.1 176.5 2.2 7.4 0.256 *Thecrystalline size L of the intermediate products.

These samples were made using the same double firing method, the onlydifference being the different sintering temperature conditions in the1^(st) sintering, which results in a different crystalline size andlattice strain of the lithium deficient sintered precursors. Whenfabricated at a low 1^(st) sintering temperature of 820° C., NMC E3 hasa crystalline size of 33.8 nm. When the sintering temperature goes downby 100° C., the crystalline size of NMC E4 is 31.1 nm.

EXAMPLE 5 NMC Samples Prepared Using a High Tray Load During the 2^(nd)Sintering

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ with M′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4)Co_(0.2) is prepared using the steps of Example1, except that the lithium deficient sintered precursor has a ratio ofLi:M=0.93, and is prepared with a high tray load of 7 kg during the2^(nd) sintering. The amount of blend on the tray is twice as much asthe Example 1, where it was 3.5 kg. Table 9 summarizes theelectrochemical performance and soluble base content of NMC E5.

TABLE 9 Performance of Example 5 DQ0.1 C 0.1 C QFad. 1 C QFad. Li₂CO₃Sample (mAh/g) (%/100) (%/100) (wt %) NMC E5 174.8 −0.3 4.1 0.315

The NMC E5 sample shows a higher weight percentage of lithium carbonatethan other NMC samples because the NMC material is prepared with a hightray load. Nevertheless, it exhibits good electrochemical performancesas it does not show a loss of discharge capacity per cycle at 0.1C after25 cycles, and 0.041% loss for 1C. Therefore, by using the double firingmethod according to the invention, it is possible to obtain anickel-excess NMC powder having good cycling stability even inlarge-scale manufacturing.

EXAMPLE 6 Al Coated NMC Samples

An Al coated NMC sample NMC E6 is obtained using the steps in Example 1and “Surface Coating Example 1”. After blending with a nanometricalumina powder (2 g alumina per kg NMC), homogeneous blending in aHenschel type mixer, and sintering at 750° C. (the dwell time beingaround 5 hrs), NMC powder is surrounded by an Al layer on the surface.

EXAMPLE 7 Al/F Coated NMC Samples

An Al/F coated NMC sample NMC E7 is obtained using the steps in Example1 and “Surface Coating Example 2”: 1 kg of NMC powder is filled into amixer (in the example a 2L Henschel type Mixer), 2 g of fumed alumina(Al₂O₃) nano-powder and 3 g polyvinylidene fluoride (PVDF) powder isadded as well. After homogeneously mixing (usually 30 mins at 1000 rpm),the mixture is sintered in a box furnace in an oxidizing atmosphere. Thesintering temperature is 375° C. and the dwell time is ˜5 hrs. As aresult, the NMC powder has an Al/F layer on the surface.

FIG. 8 shows the coin cell results of the NMC E6 and NMC E7 samples,where the square symbol is for NMC E6 and the circle symbol is for NMCE7. The cycling stability of NMC E6 is excellent, and from the Figure,it can be observed that the NMC E7 has an even better discharge capacityand cycling stability than NMC E6. Moreover, the NMC E7 has a lowlithium carbonate content of 0.104 wt % while the content in the NMC E2is 0.196 wt %. Therefore, the Al/F layer reduces the amount of solublebase in the final product and stabilizes the surface against unwantedside reactions between the NMC surface and electrolytes, which resultsin enhanced cycling performance.

EXAMPLE 8 Al Coated NMC Samples Prepared Using Roasted Transition MetalSource

An Al coated NMC sample NMC E8 is obtained using the steps in Example 2and “Surface Coating Example 1” (as in Example 6). After blending withaluminum source and sintering, NMC powder is surrounding by Al layer onthe surface.

EXAMPLE 9 Al/F Coated NMC Samples Prepared Using Roasted TransitionMetal Source

An Al coated NMC sample NMC E9 is obtained from the Example 2 and“Surface Coating Example 2” (as in Example 7). After blending with thealuminum source and fluorine-containing polymer and subsequentsintering, NMC powder has an Al/F layer on the surface.

FIG. 9 shows the coin cell results of the NMC E8 and NMC E9 samples,where the square symbol is for NMC E8 and the circle symbol is for NMCE9. The cycling stability of NMC E8 is excellent, and from the Figure,it can be observed that the NMC E9 has an even better cycling stabilitythan NMC E8. The NMC product prepared by the preroasted transition metalsource further shows a reduced content of soluble base in the final NMCproduct by Al/F layer and improved stability between the NMC surface andthe electrolyte. Accordingly, it shows an enhanced cycling performance.

EXAMPLE 10 NMC Samples Prepared Using Various Air Flow Conditions Duringthe 1st Sintering

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ with M′=Ni_(0.4)(Ni_(1/2) Mn_(1/2))_(0.4) Co_(0.2) is prepared using the steps inExample 2. In this example, in the rotary furnace for the 1^(st)sintering, an air flow condition is set in the range of 0.5 to 2 m³/kg.In case of 0.5 m³/kg air flow, the sample is labelled NMC E10.1. Whenthe air flow is 1.0 and 2.0 m³/kg, the samples are labelled as NMC E10.2& 10.3 respectively. FIG. 10 shows the total base content in the finalNMC products. As shown in the figure, when the air flow during the firstsintering is 0.5 m³/kg, there is a large variation of total base amount,which can be attributed to the low air flow that seems not enough toevacuate the produced CO₂ gas completely, and results in larger amountsof lithium carbonate in the final product. When the air flow is 1.0m³/kg and 2.0 m³/kg, it has a better variation of total base. Therefore,in order to minimize the formation of soluble base, it is preferred touse an air flow of 1.0 m³/kg or more.

FIG. 11 indicates the Li:M stoichiometric ratio in various samples ofthe product after the 1^(st) sintering. In case of a slow air flow suchas 0.5 m³/kg, the incomplete removal of CO₂ gas during the preparationcauses an inhomogeneous composition. When the air flow is 1.0 m³/kg ormore, a better variation of Li:M ratio is achieved. NMC E10.3 exhibitsthe best variation of Li:M ratio after 1^(st) sintering. Therefore, toproduce a high quality of NMC, an air flow of 2.0 m³/kg is even better.

EXAMPLE 11 Al/Sulfate Coated NMC Sample

An NMC powder with formula Li_(1.017)M′_(0.983)O₂ withM′=Ni_(0.45)(Ni_(1/2) Mn_(1/2))_(0.35)Co_(0.2) is prepared using thesteps of Example 1, except that a mixed nickel-manganese-cobaltoxyhydroxide (M′O_(0.39)(OH)_(1.61), whereM′=Ni_(0.625)Mn_(0.175)Co_(0.2)) is used as a precursor (M1) and thelithium deficient sintered precursor has a ratio Li:M=0.883. In the1^(st) sintering step, the mixture of transition metal source andLiOH.H₂O is sintered at 820° C. in a rotary furnace for 2 hours(residence time) and 0.628 rpm of rotation speed, under dry air with aflow rate of 1.67 m³/kg. The crystalline size of the lithium deficientsintered precursor after the 1^(st) sintering is 26.2 nm. The 2^(nd)sintering is conducted at 845° C. for 10 hours under a dry airatmosphere in a tray based furnace. The dry air is continuously pumpedinto the equipment at a flow rate of 40 L/hr. The sintered NMC productis labeled NMC E11.1.

The NMC product after the 2^(nd) sintering is blended with coatingsources using the steps of “Surface Coating Example 3”. First, the NMCpowder is blended with 1.2 wt % of sodium persulfate (Na₂S₂O₈) and 0.2wt % aluminum oxide (Al₂O₃) in a Henschel Mixer® for 30 minutes. Theblend is heated at 375° C. for 5 hours under air. The final productcarries a coating comprising LiNaSO₄ and Al₂O₃, and is named NMC E11.2.Table 10 summarizes the electrochemical performance and soluble basecontent of NMC E11.1 and E11.2.

TABLE 10 Performance of Example 11 DQ0.1 C 0.1 C QFad. 1 C QFad. Sample(mAh/g) (%/100) (%/100) Li₂CO₃ (wt %) NMC E11.1 180.6 −1.04 3.41 0.301NMC E11.2 184.6 0.88 1.44 0.143

These examples have a high discharge capacity because of the highNi-excess of 0.45. The Al/Sulfur coating on the NMC sample reduces thesoluble base content and shows improved battery properties, such ashigher discharge capacity and cycling stability.

The invention claimed is:
 1. A crystalline precursor compound formanufacturing a lithium transition metal based oxide powder usable as anactive positive electrode material in lithium-ion batteries, theprecursor having a general formulaLi_(1−a)((Ni_(z)(Ni_(1/2)Mn_(1/2))_(y)Co_(x))_(1−k)A_(k))_(1+a) O₂,wherein x+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and0.03≤a≤0.35, wherein the precursor has a crystalline size L expressed innm, with 15≤L≤36.
 2. The crystalline precursor compound of claim 1,having a Li₂CO₃ content <0.4 wt %.
 3. The crystalline precursor compoundof claim 1, wherein 0.35≤z≤0.50 and 0.05≤a≤0.30.
 4. The crystallineprecursor compound of claim 1, wherein the precursor has an integratedintensity ratio I003/I104<1, wherein I003 and I104 are the peakintensities of the Bragg peaks (003) and (104) of the XRD pattern of thecrystalline precursor compound.
 5. The crystalline precursor compound ofclaim 1, wherein the precursor has an integrated intensity ratio1003/1104<0.9.
 6. The crystalline precursor compound of claim 1, whereinthe precursor has a ratio R of the intensities of the combined Braggpeak (006, 102) and the Bragg peak (101) with R=((I006+I102)/I101) and0.5<R<1.16.
 7. The crystalline precursor compound of claim 1, whereinthe precursor has a crystalline size L expressed in nm, with 25≤L≤36. 8.A method for preparing a positive electrode material having a generalformula Li_(1+a′) M′_(1−a′)O₂, withM′=(Ni_(z)(Ni_(1/2)Mn_(1/2))_(y)Co_(x))_(1−k)A_(k), wherein x+v+z=1,0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and 0.01≤a′≤0.10, themethod comprising the steps of: providing a M′-based precursor preparedfrom co-precipitation of metal salts with a base; mixing the M′-basedprecursor with one of LiOH, Li₂O or LiOH.H₂O, thereby obtaining a firstmixture, whereby a Li to transition metal ratio in the first mixture isbetween 0.65 and 0.97, sintering the first mixture in an oxidizingatmosphere in a rotary kiln at a temperature between 650 and 850° C.,for a time between 1/3 and 3 hrs, thereby obtaining the lithiumdeficient precursor powder according to claim 1, mixing the lithiumdeficient precursor powder with one of LiOH, Li₂O or LiOH.H₂O, therebyobtaining a second mixture, and sintering the second mixture in anoxidizing atmosphere at a temperature between 800 and 1000° C., for atime between 6 and 36 hrs.
 9. The method according to claim 8, whereinthrough the rotary kiln an air flow is applied between 0.5 and 3.5m³/kg.
 10. The method according to claim 8, wherein the step ofsintering the second mixture is performed in a tray conveyor furnacewherein each tray carries at least 5 kg of mixture.
 11. The methodaccording to claim 8, wherein between the step of providing a M′-basedprecursor and the step of mixing the M′-based precursor with one ofLiOH, Li₂O or LiOH.H₂O, the M′-based precursor is subjected to aroasting step at a temperature above 200° C. in a protective atmosphere.12. The method according to claim 11, wherein the transition metals inthe M′-based precursor have a mean oxidation state >2.5 and wherein theprecursor has a content of H₂O<15 wt %.
 13. The method according toclaim 11, wherein the transition metals in the M′-based precursor have amean oxidation state >2.7 and wherein the precursor has a content ofH₂O<5 wt %.
 14. A method for preparing a positive electrode materialcomprising a core material having a general formula Li_(1+a′)M′_(1−a′)O₂, with M′=(Ni_(z)(Ni_(1/2)Mn_(1/2))_(y)Co_(x))_(1−k)A_(k), whereinx+y+z=1, 0.1≤x≤0.4, 0.25≤z≤0.52, A is a dopant, 0≤k≤0.1, and0.01≤a′≤0.10, and a coating comprising a metal M″, the method comprisingthe steps of claim 8 for obtaining the core material, and additionallythe steps of either: A1) providing a third mixture comprising the corematerial and a compound comprising M″, and A2) heating the third mixtureto a sintering temperature between 600° C. and 800° C.; or B1) providinga fourth mixture comprising the core material, a fluorine-containingpolymer and a compound comprising M″, and B2) heating the fourth mixtureto a sintering temperature between 250 and 500° C., or C1) providing afifth mixture comprising the core material, an inorganic oxidizingchemical compound, and a chemical that is a Li-acceptor, and C2) heatingthe fifth mixture at a temperature between 300 and 800° C. in an oxygencomprising atmosphere.
 15. The method according to claim 14, wherein thecompound comprising M″ in one of steps A1) or B1) is one or more of anoxide, a sulfate, a hydroxide or a carbonate, and M″ is one or more ofthe elements Al, Ca, Ti, Mg, W, Zr, B or Si.
 16. The method according toclaim 15, wherein the compound comprising M″ is a nanometric aluminapowder having a D50<100 nm and a BET≥50 m²/g.
 17. The method accordingto claim 14, wherein the fluorine-containing polymer in step B1) is oneof a PVDF homopolymer, a PVDF copolymer, a PVDF-HFP polymer (hexa-fluoropropylene) or a PTFE polymer, and wherein the amount offluorine-containing polymer in the fourth mixture is between 0.1 and 2wt %.
 18. The method according to claim 14 comprising steps C1) and C2),wherein M″=Li, and in step C1) the inorganic oxidizing chemical compoundis NaHSO₅, or one of a chloride, a chlorate, a perchlorate or ahypochloride of one of potassium, sodium, lithium, magnesium or calcium,and the Li-acceptor chemical is one of AlPO₄, Li₃AlF₆ or AlF₃.
 19. Themethod according to claim 14 comprising steps C1) and C2), whereinM″=Li, and in step C1) both the inorganic oxidizing chemical compoundand the Li-acceptor chemical are the same compound, being one ofLi₂S₂O₈, H₂S₂O₈ or Na₂S₂O₈.
 20. The method according to claim 14comprising steps C1) and C2), wherein in step C1) a nanosized Al₂O₃powder is provided as a further Li-acceptor chemical.